Optical modules are configured by combinations of optical transmitters that transmit optical signals and/or optical receivers that receive optical signals. An optical transmitter is equipped with a transmitter optical subassembly (TOSA).
The TOSA of a 10-mega optical module such as XFP and SFP+ has one laser diode (LD) that outputs an optical signal from the forward end and one light receiving device (PD) that monitors the optical power of the optical signal output from the reverse end of the LD. Based on the optical signal detected by the PD, a control unit monitors the optical power of the optical signal output by the LD and performs auto power control (APC) with respect to the optical power.
In recent years, the TOSA of a 100-giga optical module such as a CFP and QSFP28, for example, includes in a housing, an array of 4 adjacent LD operating at 25 Gbps and an array of 4 adjacent PDs, respectively corresponding to the LDs. With this structure, a PD 1 receives not only light output from the reverse end of an LD 1 but also a portion of the light output from the reverse ends of other adjacent LDs (LD2, LD3, and LD4), whereby optical crosstalk occurs. Similarly, at PDs 2 to 4, optical crosstalk of light from other LDs occurs.
The amount of crosstalk received by the PDs differs according to the distance to the reverse end of the LD that is the source of the crosstalk and the optical power of the crosstalk source. Further, the temperature environment during operation of the LDs varies. In the optical power monitoring and APC of the LD, the amount of crosstalk becomes errant and as a result, the optical monitoring and APC cannot be performed correctly.
To prevent such effects of crosstalk, for example, various structures are conceivable including a structure that physically blocks the light that comes from other LDs and becomes a crosstalk component, by an opening or partition disposed at the forward end of the PDs; a structure that collects the light via a lens disposed between an LD and a PD; and structure in which a wavelength filter is disposed. A structure that uses arrangement to reduce the effects of crosstalk may be considered such as that in which the LDs are positioned to be away from each other.
A multiplexed-channel transmitter optical integrated circuit has been disclosed that performs low-frequency tone modulation on the LD output of each channel as channel leveling or tagging information, and includes PDs that detect the rear light and LDs, respectively corresponding in number to the number of channels (for example, refer to Published Japanese-Translation of PCT Application, Publication No. 2009-500833).
Nonetheless, in a structure that physically blocks crosstalk light, a member such as a partition or opening has to be disposed and consequently, the size of the TOSA cannot be reduced and manufacturing cost increases. Further, with the technology using low frequency tone modulation, when output equalization of the signal channels is performed, crosstalk components cannot be extracted and the effects of crosstalk between channels on optical power monitoring and APC cannot be resolved.